Public address system



April 8, 1958 A. w. BLOW 2,830,117

- PUBLIC ADDRESSSYSTEM Filed Dec. 7, 1953 FIG. 2

INVENTOR.

ANGUS W. BLOW AGENT nite States acin PUBLIC ADDRESS SYSTEM Angus W.Blow, Rochester, N. Y., assignor, by mesne assignments, to GeneralDynamics Corporation, a corporation of Delaware Application December 7,1953, Serial No. 396,667 6 Claims. c1. 179-1 Nice Referring now to Fig.1, there are provided, as in a conventional public address system,amplifying means. 1 and an output transducer, such as loudspeaker 2.Those skilled in the art can readily appreciate that any conventionalamplifier system may be used, and may accommodate more than the singleoutput transducer shown through well-known circuit configurations. a

Line 3 is the line mentioned above. Input transducers 4 may be locatedat various points in the public address system, but may be connected toline 3 by suitable selective connecting means, such as switch means 5.Again, those skilled in the art can appreciate that a greater or lessernumber of input transducers than shown may be employed according to myinvention.

In accordance with my invention, I connect, in the I embodimentof Fig.l, non-linear impedance means 6.in

such as microphones or loudspeakers used as microphones, may beselectively connected to an amplifier input line at will for paging overan output transducer, such as a loudspeaker. p r

It most often occurs in arrangements of this type that the line forconnecting the input transducers to the input of the amplifying means isso long that whenever the line is floating, that is, when no inputtransducer is connected to the line for paging purposes, a variety ofspurious and stray interference noises and signals, such as induced hum,static and crosstalk, for example, may be picked up by the line andconveyed to the amplifier. As a result, all within hearing of the outputtransducer are bothered by annoying sounds. The noises and extraneoussignals are particularly bothersome if the amplifier is operated at ahigh level of gain, which is usually the case.

series with line 3. Non-linear impedance means 6 is preferablysubstantially resistive in character. Thus, means 6 may be a varistor,thermistor or other nonlinear impedance means having a high internalimpedance at low voltages and a low impedance when the current is high.Non-linear impedance means 6 is preferably included in each side of line3, although beneficial results will be obtained if only one side of line3 has non-linear impedance means 6 placed in series with it.

There is preferably provided in the embodiment of V Fig. 1 transformingmeans 7 for coupling non-linear im- Prior to my invention, the only waysof preventing inter- I ference of this type have been relativelyexpensive, since they generally require the use of a relay to ground theamplifier input, etc.

It is accordingly an object of my invention to provide new and usefulmeans for public address systems for limiting extraneous noises pickedupby an amplifier input line to which, upon occasion, no input transduceris connected.

It is also an object of my invention to provide in a public addresssystem, simple, cheap and easy to install extraneous-noise-reducingmeans.

In general, I accomplish these and. other'objects of my invention byproviding a circuit for connecting the input transducer to theamplifying means, and by providing nonlinear impedance means and asource of inaudible current connected in the circuit such that currentfrom the source flows through the non-linear impedance means only whenan input transducer is connected to the line. The amplifying means, uponlooking back into the line, consequently sees a very. high impedancewhen there is no transducer connected to the line, but sees a lowimpedance when a transducer is connected to the line. Signals from aninput transducer connected to the input line therefore reach theamplifier input with a negligible amount of attenuation.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to theaccompanying drawing in which:

Fig. 1 is a schematic wiring diagram of one preferred embodiment of myinvention; and

Fig. 2 is a schematic wiring diagram of another preferred embodiment ofmy invention.

is. posite those connected to source 11.

pedance means 6 to the input of amplifying means 1. Transforming means 7may have a pair of primaries 8 and a secondary 9. The latter may beconnected to the input of amplifier 1. Resistor 10 may be shunted acrossthe leads of secondary 9 to provide a load for transforming means 7.This enables the non-linear impedance means to work into an impedancecomparable to its own when current from source 11 is flowingtherethrough.

Further in accordance with my invention, I provide-a source of inaudiblecurrent 11, by which I mean that the frequency of the current is outsidethe range of audible reproducibility by transducer 2 or other outputtransducers. The source preferably furnishes direct current 1: and maybe a battery as indicated, although other sources,

such as a supersonic oscillator, might be employed. In any case, primarywindings 8 are preferably connected in series through source 11, thenon-linear impedance means being connected to the sides of primarywindings 8 op-- The embodiment of my invention shown in Fig. 1 operatesas follows: When no transducer 4 is connected to line 3, no currentflows through nonlinear impedance means 6 because the circuit for source11 is incomplete. Non-linear impedance means 6 therefore has a highvalue of impedance, and as a result, any voltage picked up by line 3 isprevented from reaching transforming means 7. When one of switch means 5is closed, however, an obvious circuit is established for current source11 to flow through non-linear impedance means 6. Because of theresultant current flow, non-linear impedance means 6 reaches anappreciably lower value of impedance, and audio-frequency currentsgenerated at the particular input transducer 4 connected to line 3 maythereupon reach input transforming means 7 with a minimum ofattenuation. Thus, in accordance with my invention, I am enabled toblock undesired noises from amplifier 1 when no paging is taking placefrom any input transducer 4.

In the embodiment of Fig. 2, there is likewise provided amplifying means1 and one or more output transducers of which only loudspeaker 2 isshown. There are also one or more input transducers 4, each selectivelyconnectiole by its own switch means 5 to line 2.

In accordance with my invention, I connect to the arrangement ofFig. 2 asource of inaudible current 11 as in the case of Fig. 1, together withnon-linear impedance means 12 "The latter may be tungsten-filamentlamps,

which in one pilot lamp version exhibit a hot-to-cold resistance ratioof 14 to 1. Signals may be transferred from linefi to theinppt ofamplifying means 1 by suitable means suchas coupling capacitorslmSuitable imped anc emeans such as linear resistors 14 are preferablyconnected in series with line 3. The impedance of the nonlinear means ispreferably comparable to that of nonlinear impedance means 12 when nocurrent from source 11 is flowing therethrough.

The embodiment of Fig. 2 operates as follows: when no input transduceris connected to line 3, lamps I2 are cold,and therefore have a lowresistance. Extraneous voltages picked upon line 3 are therefore shuntedto ground, represented by the symbol 15. Under these cit cumstances, theextraneous noises cannot reach the input of amplifier 1. When an inputtransducer 4 is connected to line 3 through the closure of a switch 5,however, an obvious circuit is completed for causing the flow of currentfrom inaudible current source'll through lamps 12. The current flowraises the resistance of the lamps 12., thereby decreasing the shuntingeifect of lamps '12 to ground on the voice currents transmitted overline 3 from input transducer 4. Paging may then take Place.

Non-linear impedance means 6 or 12, of course, may be for coupling theinput of said amplifying means to said series circuit, 'an inputtransducer and means for selectively connecting said transducer acrosssaid series'D. C. circuit whereby the current from said source flowsthrough said impedance means upon the selective completion of saidseries circuit thereby changing the value of said impedance means. p

3. In a public address system, the combination of a line,transformingmeans, amplifying means, means for connecting the input ofsaid amplifying means to the output of said transforming means,impedance means having selectively a first or a second value ofimpedance, said impedance means coupling said line to the input of saidtransforming means, an input transducer, and means for selectivelyconnecting said transducer to said line, and means responsive to theestablishment of said connection for selectively causing said impedancemeans to assume its second value of impedance.

' 4. In a public address system, the combination of a line, atransformer having a pair of primaries and a secondary, amplifyingmeans, means forconnecting the input of said amplifying means to thesecondary of said transformer, impedance means responsive to currentflow to have selectively different values of impedance, a

source of current, means for serially connecting said impedance means,an input transducer, and means for selectively connecting saidtransducer across said series circuit to cause current flow' throughsaid impedance selectively a high and a low value of impedance and tionswill readily occur to those skilled in the art. I

do not, therefore, desire my invention to be limited to the specific.arrangement shown and described, and I intend in the appended claims tocover all modifications within the spirit and scope of my invention.

-.What I claim is:

1. Ina public address system, the combination of a line, amplifyingmeans, means for coupling said line to the input of said amplifyingmeans, said coupling means having selectively a high or a low level ofcoupling, an input transducer, means for selectively connecting saidtransducer tosaid line, and means to cause said coupling means toselectively assume its high level of coupling in response to theconnection of said transducer to said line. i

1 2. In a public address system, the combination of a series D. C.circuit including a line, non-linear impedance means and a source ofcurrent, amplifying means, means.

means to thereby change its value of impedance.

5 The combination of claim 1 in which said coupling means includes 1non-linear impedance means having means to connect said impedance inshunt with the input of saidamplifying means and in series with saidline.

6. The combination of claim 1 in which said coupling means includesnon-linear impedance means having selectively a high and a low value ofimpedance, said means to cause the coupling means to selectively assumeits high level of coupling includes a source of current and saidcoupling means further including means for serially connecting saidsource and. said impedance,

means in series with said line and in shunt with the in- 45 put of saidamplifying means.

References Cited in the file of this patent UNITED STATES PATENTS

